Prevention Of Environmentally Induced Degradation In Carbon_epoxy

STARS Citation Tipton, Bradford, "Prevention Of Environmentally Induced Degradation In Carbon/epoxy Composite Material Via Implementation Of A Polymer Based Coati" 2008... PREVENTION O

University of Central Florida

STARS Electronic Theses and Dissertations, 2004-2019

University of Central Florida

Part of the Materials Science and Engineering Commons

Find similar works at: https://stars.library.ucf.edu/etd

University of Central Florida Libraries http://library.ucf.edu

This Masters Thesis (Open Access) is brought to you for free and open access by STARS It has been accepted for inclusion in Electronic Theses and Dissertations, 2004-2019 by an authorized administrator of STARS For more information, please contact STARS@ucf.edu

STARS Citation

Tipton, Bradford, "Prevention Of Environmentally Induced Degradation In Carbon/epoxy Composite

Material Via Implementation Of A Polymer Based Coati" (2008) Electronic Theses and Dissertations, 2004-2019 3674

https://stars.library.ucf.edu/etd/3674

PREVENTION OF ENVIRONMENTALLY INDUCED DEGRADATION

OF CARBON/EPOXY COMPOSITE MATERIAL VIA

IMPLEMENTATION OF A POLYMER BASED COATING SYSTEM

by

BRADFORD TIPTON B.S Rensselaer Polytechnic Institute, 2000

A thesis submitted in partial fulfillment of the requirements

for the degree of Master of Science

in the Department of Mechanical, Materials, and Aerospace Engineering

in the College of Engineering and Computer Science

at the University of Central Florida

Orlando, Florida

Fall Term 2008

© 2008 Bradford Tipton

ABSTRACT

As the use of fiber reinforced plastics increases in such industries as aerospace, wind energy, and sporting goods, factors effecting long-term

durability, such as environmental exposure, are of increasing interest The

primary objectives of this study were to examine the effects of extensive

environmental exposure (i.e., UV radiation and moisture) on carbon/epoxy

composite laminate structures, and to determine the relative effectiveness of polymer-based coatings at mitigating degradation incurred due to such exposure Carbon/epoxy composite specimens, both coated and uncoated, were subjected

to accelerated weathering in which prolonged outdoor exposure was simulated

by controlling the radiation wavelength (in the UV region), temperature, and humidity Mechanical test data obtained for the uncoated specimens indicated a reduction in strength of approximately 6% after 750 hours of environmental

exposure This reduction resulted from the erosion of the epoxy matrix in

additional to the formation of matrix microcracks Test data revealed that no further degradation occurred with increased exposure duration The protective coatings evaluated were all epoxy based and included two different surfacing films and a chromate containing paint primer The surfacing films were applied during initial cure of the carbon/epoxy composite laminate, and the chromate containing epoxy based paint primer was applied subsequent to curing the

carbon/epoxy composite laminate Although the chromate primer performed well

initially, degradation of the underlying substrate was detected with extended exposure durations In contrast, the surfacing films provided superior protection against environmentally induced degradation Similar degradation attributes were identified in the surfacing film as observed in the uncoated composite, but the degradation was either confined within the surfacing film layer or only penetrated the very near surface of the carbon/epoxy substrate This limited degradation results in a minimal reduction in mechanical strength

I would like to dedicate this thesis to my family and to my wife Charlene Without all of your love and support my education would not have been possible

ACKNOWLEDGEMENTS

I would like to offer special thanks to my advisor Dr Yong-ho Sohn for his patience and guidance throughout my journey through graduate school

I would also like to express my deep gratitude to my colleagues,

particularly David Podracky, Amador Motos-Lopez, Ed Jones, Nancy Kozlowski, Tom Chenock, Mike Gordon and Ed Nixon for all of their contributions to this research

Additionally, I would like to thank Yali Tang from InterCat for his most beneficial assistance in performing the SEM analysis

TABLE OF CONTENTS

LIST OF FIGURES viii!

LIST OF TABLES x!

LIST OF ACRONYMS/ABBREVIATIONS xi!

1.0! INTRODUCTION 1!

2.0! LITERATURE REVIEW 4!

2.1! Chemistry of Epoxy Polymers 4!

2.2! Environmental Degradation of Carbon/Epoxy Composites 13!

2.2.1! Degradation Due to Moisture Exposure 14!

2.2.2! Degradation Due to Ultraviolet (UV) Radiation Exposure 16!

2.2.3! Synergistic Effects of Moisture and UV Radiation 18!

2.3! Mechanisms of Degradation Induced by Exposure to Ultraviolet Radiation 20!

2.3.1! Chemical Reaction Mechanisms 20!

2.3.2! Degradation of the Epoxy Matrix as a Function of Depth 28!

3.0! EXPERIMENTAL DETAILS 30!

3.1! Testing Methodology 30!

3.2! Test Panel Fabrication 31!

3.3! Pre-Exposure Testing 35!

3.4! Accelerated Weathering Exposure Testing 37!

3.5! Post Exposure Testing 39!

3.5.1! Visual Micro-inspection 39!

3.5.2! Mechanical Testing 39!

4.0! RESULTS 42!

4.1! Weight as a Function of Environmental Exposure Duration 42!

4.2! Visual Inspection of Specimens Subjected to Accelerated Environmental Exposure 43!

4.3! Mechanical Test Results 54!

5.0! DISCUSSION 61!

5.1! Environmentally Induced Degradation in Carbon/Epoxy Composite Material 61!

5.2! Prevention of Degradation via Implementation of Polymer Based Coatings 63!

6.0! SUMMARY AND RECOMMENDATIONS 67!

7.0! APPENDIX A: RAW TEST DATA 69!

8.0! APPENDIX B: SUPPORTING DOCUMENATION 80!

9.0! REFERENCES 111!

LIST OF FIGURES

Figure 1 Representation of the Epoxy (a) and the Glycidyl (b) Groups [3] 6

Figure 2 Common Epoxy Synthesis Reaction [3] 7

Figure 3 (a) Tri-functional Epoxy; (b) Tetra-functional Epoxy [3] 9

Figure 4 Epoxy Curing Reaction with Amine Curing Agent [3] 12

Figure 5 Epoxy Crosslinking Mechanism [3] 12

Figure 6 Proposed Mechanism for Photo-Oxidation of TGDDM/DDS Epoxy Polymer Scheme 1 [10] 24

Figure 7 Proposed Mechanism for Photo-Oxidation of TGDDM/DDS Epoxy Polymer Scheme 2 [10] 25

Figure 8 Proposed Mechanism for Photo-oxidation of TGDDM/DDS Epoxy Polymer Scheme 3 [10] 26

Figure 9 Proposed Mechanism for Photo-oxidation of TGDDM/DDS Epoxy Polymer Scheme 4 [10] 27

Figure 10 Carbon/Epoxy Composite Test Panel Cure Cycle 33

Figure 11 Control Test Panels: No Environmental Exposure 34

Figure 12 Fiber Orientation for ASTM 3518 Test Specimen [11] 36

Figure 13 In-Plane Shear Test Specimen Dimensions 41

Figure 14 Percentage Weight Loss as a Function of Accelerated Environmental Exposure Duration 43

Figure 15 Bare Composite (A) No Exposure (B) 1500 Hrs Exposure 44

Figure 16 Chromate Primer Coated Composite (A) No Exposure (B) 1500 Hrs Exposure 44

Figure 17 Surfacing Film A (A) No Exposure (B) 1500 Hrs Exposure 45

Figure 18 Surfacing Film B (A) No Exposure (B) 1500 Hrs Exposure 45

Figure 19 Secondary Electron SEM Images (1000x) of Bare Carbon/Epoxy Composite (A) No Exposure (B) 750 Hrs Exposure (C) 1000 Hrs Exposure (D) 1500 Hrs Exposure 48

Figure 20 Secondary Electron SEM Image (25000x) of Bare Carbon/Epoxy Composite after 1500 Hrs of Environmental Exposure 49

Figure 21 Secondary Electron SEM Images (5000x) of Bare Carbon/Epoxy Composite (A) No Exposure (B) 750 Hrs Exposure (C) 1000 Hrs Exposure (D) 1500 Hrs Exposure 50

Figure 22 Secondary Electron SEM Images (50x) of Bare Carbon/Epoxy Composite (A) No Exposure (B) 750 Hrs Exposure 50

Figure 23 Secondary Electron SEM Image (1000x) of Chromate Primer Coated Carbon/Epoxy Composite (A) No Exposure (B) 1500 Hrs Exposure 51

Figure 24 Secondary Electron SEM Image (1000x) of Carbon/Epoxy Composite Coated With Surfacing Film A (A) No Exposure (B) 1500 Hrs Exposure 51

Figure 25 Secondary Electron SEM Image (1000x) of Carbon/Epoxy Composite Coated With Surfacing Film B (A) No Exposure (B) 1500 Hrs Exposure 52

Figure 26 Secondary Electron SEM Image (50x) of Carbon/Epoxy Composite Coated with Surfacing Film A – 750 Hrs Exposure 52 Figure 27 Cross Section Images (50x) of Carbon/Epoxy Specimens coated with (A) Surfacing Film A and (B) Surfacing Film B 53 Figure 28 Ultimate Load vs Exposure Time (A) Bare Composite (B) Chromate Primer Coated Composite (C) Surfacing Film A (D) Surfacing Film B 59 Figure 29 Ultimate Load as a Function of Coating Configuration and Exposure Duration 60 Figure 30 Ultimate Load as a Function of Coating Thickness and Exposure Duration 60

LIST OF TABLES

Table 1 Detailed Test Panel Fabrication Matrix 34!

Table 3 Mechanical Strength Values No Exposure 57

Table 2 Detailed Accelerated Weathering Test Matrix 38!

Table 4 Mechanical Strength Values 750 Hr Exposure 57

! Table 5 Mechanical Strength Data 1000 Hr Exposure 58!

! Table 6 Mechanical Strength Data 1500 Hr Exposure 58!

LIST OF ACRONYMS/ABBREVIATIONS

Spectroscopy

pre-impregnated with matrix resin

1.0 INTRODUCTION

In the most basic sense, a composite material is simply a mixture of two or more distinct solid constituents that are, in theory, mechanically separable and, when combined, produce a material with superior properties to the individual constituents alone Typically, the composite material consists of a binder or

matrix that surrounds and holds reinforcements in place The separate

characteristics of the matrix and reinforcements contribute synergistically to the overall properties of the composite material [2,3,9] This definition includes a wide assortment of materials including steel reinforced concrete, particle filled plastics, ceramic mixtures, and some alloys [3] This study focuses on a class of

composites known as fiber reinforced plastics More specifically, materials

composed of an epoxy polymer matrix reinforced with carbon fibers

The key advantage for using composite materials for structural

applications is the weight reduction realized due to the high strength-to-weight and stiffness-to-weight ratios [1] For example, in aerospace applications, weight savings on the order of 25% are generally considered to be achievable using current composite materials in place of metals [2]

In composite materials, all of the properties arise, to some extent, from the

contributes different attributes to the overall composite material performance The principal role of the reinforcement is to provide mechanical properties such as

strength and stiffness and to carry the load imposed on the composite structure

In structural applications, 70% to 90% of the load is carried by the reinforcements

together, transfer loads to and between the fibers, and to protect the fibers from self-abrasion and externally induced scratches The matrix also protects the fibers from environmental degradation, which can lead to embrittlement and premature failure [1]

The use of fiber-reinforced plastics has steadily increased in markets such

as aerospace, wind energy, and sporting goods In the past 15 years, the market demand for glass-reinforced plastics has grown by 50% and the market demand

these fiber reinforced plastic composite materials increases, factors effecting long-term stability and durability, such as environmental exposure, may become

a significant concern in the industries where these materials are utilized

Previous research has determined that exposure to environmental factors such

as Ultraviolet (UV) radiation, moisture, and temperature results in a reduction in matrix dominated properties, resulting in a decrease in the overall performance of the composite material [1,7,8]

The primary objectives of this study were to examine the effects of

prolonged environmental exposure (specifically UV radiation and moisture) on carbon/epoxy composites and to investigate the effectiveness of various polymer based coatings at preventing composite substrate degradation In order to

simulate extensive outdoor environmental exposure, carbon/epoxy composite panels, each with a different coating, were subjected to accelerated

environmental weathering After exposure, the panels underwent visual inspection and mechanical testing to determine their load carrying capability These results were compared with unexposed control specimens to determine the extent of degradation and the performance of the protective coatings

micro-2.0 LITERATURE REVIEW

2.1 Chemistry of Epoxy Polymers

In composite structures designed for low temperature applications (less than 200!F) the most widely used polymer matrix materials are epoxies

Generally considered the workhorse of the composites industry, epoxies provide outstanding chemical resistance (i.e fluids and solvents), excellent adhesion strength to fibers, and superior dimensional stability Epoxies are also favored due to their low cure shrinkage, long shelf life, and lack of void forming volatiles [1]

In order to obtain a better understanding of the specific degradation

mechanisms induced by exposure to UV radiation, a basic knowledge of epoxy chemistry must first be attained Epoxy resins belong in a class of polymers known as thermosets Thermosetting resins, which are usually liquids at room temperature, are characterized by the ability to form bonds between the

molecules of individual chains, also known as crosslinks This is made possible

by certain molecules on the polymer chains that can be activated to form reaction sites The formation of these crosslinks restricts the movements of the polymer chains, resulting in increased stiffness, strength, and temperature resistance Once the crosslinks are formed during cure, these materials cannot be melted and will degrade when exposed to extreme temperatures The other major class

of polymer resins are known as thermoplastics Contrary to the thermosets, these resin systems, usually solids at room temperature, do not form crosslink bonds and, therefore, can be melted and reformed Thermoplastics provide improved toughness over thermosets [3]

Epoxies contain a high percentage of aromatic molecules, which are characterized by the presence of variations of the aromatic functional group In the most basic form, the aromatic group is a cyclic hydrocarbon consisting of six carbon atoms each including one hydrogen atom When no other polymer groups are attached, it is referred to as benzene The aromatic group imparts strength and stiffness to the polymer chain and also increases temperature resistance On the other hand, aliphatic compounds are characterized by their complete lack of aromatic content and an increased presence of straight chain polymers Aliphatic polymers typically exhibit improved flexibility, toughness, and resistance to

weathering [3]

The basic structure of an uncured epoxy resin consists of two parts: a three-member ring epoxy group (epoxy ring) also known as an oxirane group and the rest of the polymer chain The epoxy group is considered an “active site” because it is the location where crosslinking occurs The epoxy group also gives rise to many of the characteristic properties observed with epoxies In the

complete epoxy resin structure, the epoxy ring will be attached to another organic group in the polymer chain, either directly or via an intermediate carbon atom, known as a bridge In the latter case, the epoxy group is then referred to as a

glycidyl (reference Figure 1) [3]

Figure 1 Representation of the Epoxy (a) and the Glycidyl (b) Groups [3]

The most common method of producing uncured epoxy resin is via the condensation polymerization reaction of bisphenol A with epichlorohydrin This reaction bonds the glycidyl groups to both ends of the aromatic bisphenol A compound (Reference to Figure 2) The epoxy nomenclature is derived from the various components contained within the polymer resin For example, the epoxy polymer created per the reaction in Figure 2 is referred to as DGEBPA, or

“DiGlycidyl Ether of BisPhenol A” This is in reference to the two glycidyl groups

(di-glycidyl) attached to the bisphenol A polymer via an ether linkage (R-O-R’) [3]

Figure 2 Common Epoxy Synthesis Reaction [3]

The other major component of the epoxy resin is the remaining polymer chain In figure 2, this is represented by the bisphenol A portion of the DEGBPA molecule The particular molecule chosen for this portion of the resin and the

number of repeat units (represented by “n”) can have a significant effect on the final resin characteristics, and, ultimately, the final properties of the cured

polymer For instance, increasing the number of bisphenol A units included in the resin molecule depicted in figure 2, will result in an increased resin viscosity and heat distortion temperature Besides changing the number of polymer units, different types of polymers can be utilized to increase the functionality of the epoxy resin Figure 3 illustrates examples of tri-functional and tetra-functional epoxy resins created by utilizing different aromatic polymer linkages These resins have an increased number of active sites available for crosslinking

reactions Upon cure, the ability for increased crosslinking will lead to a material with higher strength, stiffness, and temperature resistance [3]

(a)

Figure 3 (a) Tri-functional Epoxy; (b) Tetra-functional Epoxy [3]

The methods used for synthesis of epoxy resins are quite different from those utilized to crosslink and cure them The crosslink reaction in epoxy resins is based upon the opening of the epoxy ring by a reactive group on the end of another molecule known as the curing agent or hardener A typical curing agent

each end The presence of the amine reactive groups on either end allows the curing agent to react with two epoxy groups on two different molecules, thus linking them together [3]

The ring-opening reaction is initiated when the reactive portion of the curing agent comes into close proximity with the epoxy ring The nitrogen atom has a slightly negative charge and seeks the slightly positive charge of the

carbon atom in the epoxy ring The end carbon of the epoxy ring, which is the terminal carbon of the chain, is usually the more accessible of the two epoxy-ring carbons It is therefore the atom that usually reacts with the nitrogen The

nitrogen forms a bond with the carbon, breaks open the epoxy ring, and loses a hydrogen atom in the process This hydrogen atom, which is slightly positive, will then bond to the oxygen that was initially part of the epoxy ring

This hydroxyl group (OH-) is capable of reacting with other epoxy rings in the crosslinking reaction, which can establish a chain reaction referred to as homopolymerization In this instance the curing agent can be viewed as an initiator However, the curing agent and homopolymerization reactions will generally occur together to complete crosslinking during cure Figures 4 and 5 illustrate the ring-opening reaction mechanism and the epoxy crosslinking mechanism, respectively [3]

Figure 4 Epoxy Curing Reaction with Amine Curing Agent [3]

Figure 5 Epoxy Crosslinking Mechanism [3]

2.2 Environmental Degradation of Carbon/Epoxy Composites

As mentioned in the introduction, there are several inherent advantages to using composite materials for structural applications (i.e, high strength and

stiffness to weight ratios) Despite these benefits, there are concerns regarding the overall long-term durability of these materials, especially as related to their capacity for sustained performance under harsh and changing environmental conditions [5] Composite structures must be designed to withstand the great diversity of environments encountered in a variety of operations For instance, in aerospace applications, environmental effects, including combinations of heat, cold, moisture, lightening strikes, UV radiation, fluids, and fuels, can reduce mechanical properties to varying degrees, depending on the composite system and the particular design application [1]

Although the most important contribution to the material strength is that of the fiber, the overall performance of the composite structure also depends greatly

on the properties of matrix in addition to the quality of the fiber-matrix bond The matrix, in addition to binding the fibers together and protecting them from

environmental effects, serves to transfer applied structural loads to the fibers The fiber-matrix interface governs these load transfer characteristics and

contributes to the overall damage tolerance of the structure [1,5]

The composite matrix is generally the component most vulnerable to environmental attack, with UV light and moisture being two of the primary

environmental factors contributing to material degradation In general, matrix degradation induced by environmental exposure is manifested as matrix cracking and erosion that leads to a reduction in matrix dominated properties

Consequently, matrix-dominated properties are of particular concern with regard

to environmental exposure of carbon/epoxy composites [1,2,7]

Previous research determined that a carbon/epoxy laminate exposed to

500 hours of UV exposure, would see a reduction in transverse tensile strength

of 9% and a laminate exposed to 500 hours of moisture via condensation would see a reduction of 20% When laminates are exposed to both UV radiation and condensation, either sequentially or in a cyclical manner, the combined effects can produce even greater degradation [5] The synergistic effects of UV radiation and condensation are discussed further in subsequent sections

2.2.1 Degradation Due to Moisture Exposure

The absorption of moisture by the epoxy matrix as a result of

environmental exposure can have detrimental effects on the overall mechanical properties of the carbon/epoxy composite structure The moisture diffuses into the matrix, which leads to dilatation expansion and also chemical changes such

as plasticization and hydrolysis [1,7]

In degradation by moisture ingress, the controlling factor is the diffusion constant of water vapor As water is a very polar molecule, the diffusion

mechanism involves hydrogen bonding with polar sites in the polymer molecule Epoxy resins are the most polar of the normal resins as they contain hydroxyl groups, ether groups, and C-N bonds Thus, water permeability is highest for epoxy resins This can result in both reversible and irreversible damage to the epoxy matrix Plasticization is usually reversible upon desorption of moisture, while hydrolysis of chemical bonds results in permanent irreversible damage Moisture desorption gradients may induce microcracking as the surface desorbs and shrinks, putting the surface in tension If the residual tension stress at the surface is beyond the strength of the matrix, cracks occur Additionally, moisture wicking along the fiber-matrix interface can degrade the fiber-matrix bond, resulting in loss of microstructural integrity [1,2,5]

All of these factors manifest in a decrease in matrix-dominated properties such as compressive strength, interlaminar shear strength, fatigue, and impact tolerance Although the carbon fibers do not absorb moisture and their physical properties remain unaffected, the deterioration of the matrix alone is sufficient to cause a decrease in performance and overall reliability [1,5]

2.2.2 Degradation Due to Ultraviolet (UV) Radiation Exposure

In addition to degradation due to moisture absorption, the epoxy matrix in carbon/epoxy composite structures is also susceptible to attack by incident light The most important interaction of light with the polymer matrix is from the UV component of light The UV components of solar radiation incident on the earth surface are in the 290-400 nm band The energy of these UV photons is

comparable to the dissociation energies of polymer covalent bonds, which are typically 290-460 kJ/mole Therefore, the interactions between this UV light and the electrons are strong, often resulting in excitation of the electrons and a

resultant breaking of the bond Hence, UV light can degrade polymers [3,5]

The nature of the atoms in polymer matrix has some effect on the

tendency of the electrons to become excited by the UV light and degrade

Generally, aromatic polymers are more easily degraded by UV light then are aliphatic polymers All resins containing aromatic groups can absorb sufficient UV radiation to cause bond dissociation Of the typical resins used in composite structures, phenolics are most sensitive, followed by epoxy resins The high aromatic content common to most high-performance epoxies makes them

particularly susceptible to UV radiation induced degradation [2,3]

The UV photons absorbed by polymers result in photo-oxidative reactions that alter the chemical structure resulting in material deterioration The chemical reactions typically cause molecular chain scission and/or chain cross-linking Chain scission lowers the molecular weight of the polymer, giving rise to reduced heat and strength resistance Chain cross-linking leads to excessive brittleness and can result in microcracking Previous research discovered that exposure of a carbon/epoxy laminate to UV radiation for as little as 500 hrs results in the formation of microcracks, which lead to a reduction in matrix-dominated properties This was likely caused by embrittlement of the polymer matrix due to increased crosslinking resulting from photo-oxidation reactions induced by UV exposure More detailed discussion of UV radiation degradation mechanisms is included in subsequent sections [5]

Some polymers, including epoxies, exhibit a color change when exposed

to UV radiation In addition to inducing chain scission and increasing crosslink density, photo-oxidative reactions can also result in the production of

chromophoric chemical species Chromophores are simply molecules that transmit and absorb light These chromophores, may impart discoloration to the polymer, if they absorb visible wavelengths Furthermore, an autocatalytic

degradation process may be established if chromophores produced also absorb

UV radiation [5]

2.2.3 Synergistic Effects of Moisture and UV Radiation

While the previous sections have focused on the individual degradation effects due to UV radiation and moisture exposure, these environmental factors can act in conjunction to further enhance the degradation of the carbon/epoxy composite structure [5]

Exposure to UV radiation results in the formation of a thin surface layer of chemically modified epoxy Subsequent water condensation leaches away

soluble UV degradation products, which exposes a fresh layer that can once again be attacked by UV radiation In this manner, a repetitive process is

established that leads to significant erosion of the epoxy matrix Furthermore, it is also conceivable that the presence of absorbed water molecules in the epoxy matrix can enhance the photo-oxidation reactions due to increased availability of

OH- and H+ ions This would increase the chain scission and crosslinking

reactions occurring on the surface of the epoxy polymer, thus increasing the brittleness of the matrix These synergistic mechanisms result in more extensive microcracking and loss of fiber confinement due to matrix erosion, ultimately leading to a more significant reduction in the overall mechanical properties of the composite structure [5,7]

In research conducted by Kumar et al., carbon/epoxy laminates exposed

to cyclic exposure of both UV radiation and moisture condensation totaling 1000 hrs resulted in extensive matrix erosion, void formation, and fiber-matrix interface

debonding The epoxy rich layer on the specimen surface was completely removed and the underlying carbon fibers were exposed Furthermore,

examination of the transverse tensile strength indicated a reduction of 29% as

2.3 Mechanisms of Degradation Induced by Exposure to Ultraviolet Radiation

2.3.1 Chemical Reaction Mechanisms

exposure to UV radiation and moisture The study utilized an aluminum substrate coated with a bisphenol A based epoxy polymer with a nominal film thickness of

30 "m These specimens were subjected to accelerated weathering, consisting

of cyclic exposures to UV radiation @ 340 nm and water vapor condensation (4 hours each) The exposed specimens were examined using a combination of photo acoustic (PA) Fourier transformed infrared (FT-IR) spectroscopy, FT-IR microscopy, and Raman chemical imaging Varying the modulation frequencies utilized with PA FT-IR facilitated the determination of molecular level information

as a function of depth [6]

Examination of unexposed specimens at depths ranging from 5-24 "m, using the PA FT-IR, detected an increase in the band intensities at 3399 cm-1, which indicates an increase in -OH (hydroxyl group) content with increasing

stretching vibrations of bisphenol A epoxy polymer and N-H deformations of polyamine crosslinker, respectively Both of these functional groups are reaction

sites responsible for crosslinking reactions of epoxy polymers This indicates that the ring opening reactions of oxirane groups of bisphenol A epoxy polymer occur further away from the surface, thus resulting in the increase in –OH group

content However, the intensity of the band attributed to C=C stretching vibrations

of bisphenol A (1607 cm-1) does not change as a function of depth, indicating that bisphenol A epoxy polymer is uniformly distributed throughout the film thickness [6]

PA FT-IR spectroscopy performed on the surface of specimens exposed to

band, indicating that UV exposure in the presence of water condensation results

in the formation of hydroxyl groups on the surface An exposure time of 5 weeks also detected a decrease in intensity of the 1250 cm-1 and 1509 cm-1 bands This indicates that UV exposure further promotes crosslinking reactions on the

surface No further decrease in these band intensities was detected with

subsequent exposures past 5 weeks However, the formation of a new band at

1660 cm-1 indicates that carbonyl amide formation is taking place on the surface This band increases in intensity with continued exposure These observations indicate that crosslinking reactions are responsible for degradation for exposures

up to 5 weeks After that time, formation of amides dominates the degradation process FT-IR microscopy and Raman chemical imaging where utilized to

examine specific aspects of the degraded surface, comparing areas with and with out observed microcracking The spectra generated detected an increase in band

intensity at 1660 cm-1 and a decrease in band intensity at 1296 cm-1 (C-N

vibrations) band in the area with microcracking These bands are attributed to higher amine content, indicating that the formation of amides, via chain scission, has a greater contribution to epoxy degradation Spectra from the microcracked area also detected an increase in the 1250 cm-1 and 1509 cm-1, indicating a diminished extent of crosslinking was present [6]

demonstrated that carbon/epoxy laminates subjected to 500 hrs of UV radiation exposure displayed similar spectra when analyzed with FT-IR Specifically,

crosslink density on the surface of the epoxy A reduction in the peak at 1296

cm-1 was also observed, attributed to C-N stretching vibrations due to amide

formation This indicated the presence of chain scission reactions [5]

Both of these studies indicated that crosslinking and chain scission

mechanisms operate in a competing manner during the degradation process Increased crosslinking dominates in the early stages of degradation, after which carbonyl amide formation by chain scission takes over Both of these

mechanisms then result in increased microcracking and surface deterioration, ultimately reducing the mechanical strength of the composite structure [5]

FT-IR analysis conducted on tetraglycidyl-4,4’-diaminodiphenylmethane (TGDDM) epoxy resin cured with aromatic hardener 4,4’-diaminodiphenyl sulfone (DDS),

subsequent to exposure to UV radiation and humidity They concluded that photo-oxidative degradation of TGDDM/DDS could potentially involve several different mechanisms, which ultimately bring about chain-scission, leading to the formation of amide and carbonyl groups Figures 6 through 9 illustrate proposed degradation Schemes 1 through 4, respectively Scheme 1 involves scission of the carbon-nitrogen bond following hydrogen abstraction on the methylene group, ultimately resulting in the formation of an aldedhyde (carbonyl group) Scheme 2 begins with hydrogen abstraction of the CH-OH bond followed by a similar chain scission reaction at the carbon-nitrogen bond, resulting in the formation of a ketone (carbonyl group) Scheme 3 begins with the oxygen attack

of structure VII depicted in scheme 2 Chain scission at the carbon-carbon bond

structure XI, an amide linkage However, the principal route for amide formation

is proposed in scheme 4, with chain scission occurring at the carbon-carbon bond, rather than the carbon-nitrogen bond, producing amide molecules which

Figure 6 Proposed Mechanism for Photo-Oxidation of TGDDM/DDS Epoxy Polymer

Scheme 1 [10]

Figure 7 Proposed Mechanism for Photo-Oxidation of TGDDM/DDS Epoxy Polymer

Scheme 2 [10]

Figure 8 Proposed Mechanism for Photo-oxidation of TGDDM/DDS Epoxy Polymer

Scheme 3 [10]

Figure 9 Proposed Mechanism for Photo-oxidation of TGDDM/DDS Epoxy Polymer

Scheme 4 [10]

2.3.2 Degradation of the Epoxy Matrix as a Function of Depth

In addition to studying the degradation aspects of an epoxy polymer film exposed to both UV radiation and moisture, Kim et al [6] also examined the molecular level degradation as a function of depth As mentioned previously, this study involved an aluminum substrate coated with a bisphenol A based epoxy polymer with a nominal film thickness of 30 "m These specimens were

subjected to accelerated weathering, consisting of cyclic exposures to UV

radiation @ 340 nm and water vapor condensation (4 hours each) The exposed specimens were analyzed using step-scan photo acoustic (PA) Fourier

transformed infrared (FT-IR) spectroscopy Varying the modulation frequencies facilitated the determination of molecular level information as a function of depth [6]

The first portion of the study determined that increased crosslinking reactions were initially responsible for degradation However, with increased exposure time the predominate degradation mechanism was the formation of carbonyl amides The other portion of the study examined specimens exposed for a 5 weeks, utilizing the PA FT-IR, at depths of 5, 9, 18 and 24 "m To

determine the depth of degradation from the exposed surface, the specimens were examined from the substrate side Examination of the spectra indicated that